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The cover picture shows a schematic representation of the recognition principle of the ABC transporter complex TAP, which was deciphered by using combinatorial peptide libraries. By comparing individual sublibraries with the complex library, the effect of each amino acid with respect to its position in the peptide sequence can be derived. Favored or disfavored residues are colored in blue and red, respectively. The decoded recognition pattern for antigenic peptides matched the binding motif of class I MHC molecules. Based on these experiments we now begin to understand how the transport and loading machinery TAP fulfills one of the most important tasks in cellular immune surveillance: constantly supplying antigenic peptides for subsequent presentation by class I MHC molecules to cytotoxic T lymphocytes. Further details are given in the review by L. Schmitt and R. Tampé on p. 16 ff.